Feature request: add a no_delete option to the storage configuration

[KaiHerlemann]

I would like to add a no_delete option to the storage configuration.

The read_only option didn't work for our use case:

1. I want to copy the calendar entries/.ics files that are in multiple calendars A, B, C etc to one calendar X. So, in calendar A are a few one calendar entries, and copying them to calendar X with vdirsyncer is no problem.
2. Then I want to copy the calendar entries of calendar B to calendar X. This works, but the calendar entries, which were copied from calendar A to calendar X, are deleted from calendar X, because they don't exist in calendar B.
3. If the read_only option which is enabled for calendars A, B, C would be enabled for calendar X as well, it wouldn't be possible to copy any calendar entry to X.

I have implemented the function in our university without any problems. Since one should wait for the discussion in the issue tracker first, I don't make a pull request yet.

[WhyNotHugo]

I have implemented the function in our university without any problems. Since one should wait for the discussion in the issue tracker first, I don't make a pull request yet.

How did you implement this?

[KaiHerlemann]

I have implemented the function in our university without any problems. Since one should wait for the discussion in the issue tracker first, I don't make a pull request yet.

How did you implement this?

See #1090

[WhyNotHugo]

This overlaps with #1091, and I'd prefer to go with the solution described there in the long term.

I'll still have a look at #1090 in the meantime.

[lukengda]

As #1090 is now merged, this would allow to sync a calendar with rolling entries like holidays without loosing old entries if the holiday calendar is rolling and removes old entries. Does it sound right? If so, this could be added to the read-only guide in the docs?